Reductive dechlorination of chlorophenols and pentachlorophenol in anoxic estuarine sediments

The fates of three isomers of chlorophenol (CP), three isomers of dichlorophenol (DCP), and pentachlorophenol (PCP) were investigated in anoxic estuarine sediment slurries. The relative dechlorination rates of the three isomers of CP were 2-CP > 3-CP > 4-CP. All three isomers of CP were dechlorinated to phenol. The relative dechlorination rates of the three isomers of DCP were 2,5-DCP > 3,4-DCP > 3,5-DCP. All these isomers were dechlorinated to 3-CP. PCP was transformed through sequential dechlorination and the relative order of product formation was (i) 2,3,4,5-tetrachlorophenol (TeCP), (ii) 3,4,5-trichlorophenol (TCP) and (iii) 3,5-dichlorophenol (DCP). 3,4-DCP was also observed. The rates of PCP dechlorination were greatest in 3,4-DCP-adapted and 2-CP-adapted sediment slurries. The PCP-adapted sediment cultures were inhibited by the addition of molybdate, or molybdate plus sulfate, but not inhibited by the addition of sulfate or bromoethanesulfonic acid (BESA). Sulfate consumption by the cultures corresponded to the stoichiometric values expected for reductive dechlorination of PCP to DCP (PCP:SO4−2 = 1:1.5). The addition of metabolizable carbon sources to sediment slurries partially repressed PCP dechlorination.